A common grid tee construction comprises a metal strip formed into an upper bulb, a vertically extending double web and oppositely extending lower flanges. It is important for good appearance when there is no cap bridging the flanges and concealing the web elements that the spacing between these elements be uniform along the length of the web. This can be accomplished by fastening the web elements together adjacent the flanges. U.S. Pat. No. 4,489,529 to Ollinger proposes several ways to join the elements of the double web. One such proposal in this patent is to form stitches by lancing the double web elements at locations spaced along the length of the tee. A problem associated with this teaching is that the effective thickness of the web at the stitch locations is doubled. The resulting thickness variation makes it difficult to accurately hold the tee for subsequent forming and/or assembly operations during manufacture. Still further, variable thickness can present difficulties for the installer where the stitch exists or otherwise interferes at a cross tee slot.
Locating the stitches so that they do not interfere with critical parts of the tee is difficult and/or expensive where they are formed in a high speed rolling operation.
It is known to lance or stitch the double web elements in a manner where the material surrounding the lanced hole is coined to reduce the size of the hole after the lance is made to positively interlock the web elements together.
The invention provides a grid tee of the double web type in which the web elements are locked together by an integral stitch with a configuration that avoids an excessive increase in the local web thickness. The stitch is formed by lancing or shearing through the double thickness of the web to displace a slug out of the plane of the web and create a corresponding hole. The web material is coined or otherwise deformed so that the slug is unable to pass back through the hole. The material forming the slug is forced back into the hole area so that the final thickness of the web in the area of the stitch is not substantially greater than the original web thickness.
In the preferred form of the invention, the web is stitched by three stages of rolling dies that first lance the stitch slug out of the plane of the web. Thereafter, the material surrounding the stitch hole is coined to decrease the size of the hole and thereby prevent the slug from passing back through it. The slug is then rolled to substantially flatten it back into the space of the hole and coined area. Since the stitch, when completed, does not substantially increase the local thickness of the web, it does not interfere with normal manufacturing operations such as where the tee is held in a fixture for hole stamping or other finishing steps such as the assembly of an end clip. Moreover, the stitch pattern, which can have a uniform spacing between stitches even though randomly located along the length of a grid, does not interfere with the reception of transverse tee connectors in slots that happen to fall on the area of a stitch.